1. Field of the Invention
The present disclosure relates generally to circuit board signal/ground plane structures, and more particularly to impedance-tuning of a signal/ground plane structure.
2. Description of Related Art
Electronic assemblies commonly employ one or more printed circuit boards in their construction. Such circuit boards provide mounting points for electronic components and/or for sockets that allow other circuit boards, cables, or device packages to connect to the circuit board. The circuit board provides conductive traces, and possibly planar conductive regions, patterned on conductive layers sandwiched between insulating dielectric layers. A typical circuit board may contain anywhere from a few conductive layers to upwards of thirty such layers for complex systems. Conductive traces route signals (and possibly power) from one point on the circuit board to another point on the circuit board. Planar conductive regions are employed for power distribution. Planar conductive regions also serve as reference planes, which when coupled through a dielectric layer to one of the conductive traces or a differential pair of such traces, form stripline transmission lines of specific impedance. Plated through-holes (PTHs) in the circuit board can form mounting points for press-fit devices, allow for signal insertion/extraction to the internal board layers, and can also serve as layer-swapping vias that transfer a signal from a trace on one conductive layer to another trace on another conductive layer.
FIG. 1 contains a perspective view of a partial circuit board assembly 100, with the intervening dielectric layers removed for visibility. Assembly 100 includes end sections of two differential trace pairs (D1+, D1−; D2+; D2−) on two signal layers, two digital ground reference planes G1 and G2 on two plane layers, and two PTHs T1+, T1−. Differential trace D1+connects to PTH T1+ at a pad P1+, and differential trace D1− connects to PTH T1− at a pad P1−. Differential trace D2+ connects to PTH T1+ at a pad P2+, and differential trace D2− connects to PTH T1− at a pad P2−. Accordingly, PTHs T1+, T1− provide electrical continuity to transfer a differential signal from differential trace pair D1+, D1− to differential trace pair D2+, D2−. Other PTHs may transfer similar signals directly from a component mounted over or near a PTH, or a component or connector having a pin inserted into the PTH, to a conductive trace on an interior circuit board trace layer.
Each PTH extends through one or more reference planes to connect two signal layers or one signal layer to the circuit board surface components/layer. PTHs T1+ and T1− both extend through ground planes G1 and G2. At the location where a signal PTH will be fabricated, a clearance is formed in each plane layer so that the PTH will not short to the plane layer. In FIG. 1, PTH T1+ passes through a clearance C1+ on G1 and a clearance C2+ on G2; PTH T1− passes through similar clearances C1−, C2− on G1 and G2, respectively.
With some PTHs, the capacitance between the through hole and a reference plane that the through hole passes through is adjusted in one or both of two ways. The first way is to vary the size of the clearance. The amount of capacitance variation available by this method is, however, limited by the minimum clearance size that must be maintained between the PTH and the ground plane to prevent shorts. To add additional capacitance, a nonfunctional pad is formed on the reference layer, centered in the clearance, and the PTH is drilled through and then electrically connected to the dead pad, such as is shown for dead pads DP+ and DP−, in clearances C1+ and C1−, respectively. One of the inventors of the present invention has previously patented the use of such nonfunctional pads on selected ground plane layers to tailor the impedance of a through hole, see, e.g., U.S. Pat. No. 6,812,803.